It is often desirable to convert alkyl aromatic compounds through disproportionation reactions to produce disproportionation products which can include unsubstituted aromatic compounds as well as alkyl aromatic compounds, including monoalkyl aromatics and polyalkyl aromatics, and mixtures thereof. Such disproportionation reactions may be employed to produce disproportionation products comprising benzene and mixtures of alkyl and polyalkyl benzenes. While such disproportionation reactions may be employed to convert relatively high molecular weight alkyl aromatics, one important disproportionation reaction involves the disproportionation of toluene to benzene and xylene. The disproportionation of toluene involves a well known transalkylation reaction in which toluene is converted to benzene and xylene in accordance with the following reaction, which is mildly exothermic:

Mordenite is one of a number of catalysts which can be employed in the transalkylation of alkylaromatic compounds. Mordenite is a crystalline aluminosilicate zeolite having a network of silicon and aluminum atoms interlinked in its crystalline structure through oxygen atoms. For a general description of mordenite catalysts, reference is made to Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, 1981, under the heading “Molecular Sieves,” vol. 15, pages 638-643. Mordenite, as found in nature or as synthesized, typically has a relatively low silica to alumina mole ratio of about 10 or less. Such conventionally structured mordenite catalysts can be employed in the disproportionation of toluene. However, aluminum deficient mordenite catalysts having substantially lower alumina contents can also be employed in the disproportionation of toluene.
Aluminum deficient mordenite catalysts have a silica/alumina ratio greater than 10 and may sometimes range up to about 100. Such low alumina mordenites may be prepared by direct synthesis as disclosed, for example, in U.S. Pat. No. 3,436,174 to Sand or by acid extraction of a more conventionally prepared mordenite as disclosed in U.S. Pat. No. 3,480,539 to Voorhies et al. U.S. Pat. No. 3,780,122 to Pollitzer discloses the transalkylation of toluene using a mordenite zeolite having a silica/alumina ratio greater than 10 which is obtained by acid extraction of a mordenite zeolite having a silica/alumina ratio of less than 10.
The disproportionation of toluene feedstocks may be carried out at temperatures ranging from about 200° C. to about 600° C. or above and at pressures ranging from atmospheric to perhaps 100 atmospheres or above. However, the catalyst itself may impose constraints on the reaction temperatures in terms of catalyst activity and aging characteristics. In general, the prior art indicates that while relatively high temperatures can be employed for the high aluminum mordenites (low silica to alumina ratios), somewhat lower temperatures should be employed for the low alumina mordenites. Thus, where mordenite catalysts having high silica/alumina ratios have been employed in the transalkylation of alkylaromatics, it has been the practice to operate toward the lower end of the temperature range. It is also a common practice in this case to promote the catalyst with a catalytically active metallic content, as disclosed, for example, in U.S. Pat. No. 3,476,821 to Brandenburg. Metal promoters are said to substantially increase activity and catalyst life and may be incorporated by treatment of the mordenite with metal sulfides such as nickel sulfide.
Hydrogen may be supplied along with the toluene to the reaction zone. While the disproportionation reaction (1) is net of hydrogen, the use of a hydrogen co-feed is generally considered to prolong the useful life of the catalyst, as disclosed, for example, in the above-identified patent to Brandenburg. The amount of hydrogen supplied, which can be measured in terms of the hydrogen/toluene mole ratio or in terms of a standard liter of hydrogen per liter of feedstock, is generally shown in the prior art to increase as temperature increases. Normally, the hydrocarbon feedstock supplied to the toluene disproportionation reaction zone is of extremely high purity. Typically, feedstocks having a toluene content of 90-100 wt. % are supplied to the reaction zone. Usually, it is considered desirable to maintain the toluene content in excess of 99 wt. % (less than 1% impurities) for toluene disproportionation processes.